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1
Department of Clinical Radiology, Kuopio University Hospital, P.O. Box 1777,
FIN-70211 Kuopio, Finland.
2
Department of Urology, Kuopio University Hospital, FIN-70211 Kuopio,
Finland.
Received April 26, 2000;
accepted after revision June 30, 2000.
Supported by Kuopio University Hospital (EVO funding no. 5063508) and the
Radiological Society of Finland.
Abstract
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SUBJECTS AND METHODS. Forty consecutive patients with symptoms of acute flank pain underwent MR urography followed immediately by excretory urography. Heavily T2-weighted (combined thin-slice half-Fourier acquisition single-shot turbo spin-echo [HASTE] and thick-slab single-shot turbo spin-echo) and 3D FLASH sequences were evaluated separately and independently by two experienced radiologists for the presence, cause, level, and degree of obstruction. Interobserver agreement was calculated using the kappa statistic. Excretory urography and the final clinical diagnosis were used as reference.
RESULTS. Twenty-six patients were found to have unilateral
obstruction caused by ureteral stones. Both MR urography methods were
excellent for detecting obstruction. In the detection of stones 3D FLASH was
superior, with a sensitivity of 96.2% and 100% and specificity of 100% and
100% for observers A and B, respectively, compared with a sensitivity of 57.7%
and 53.8% and a specificity of 100% and 100%, respectively, for T2-weighted
sequences. The best degree of obstruction was seen with 3D FLASH, and the
interobserver agreement was excellent for stone detection (
=
0.97).
CONCLUSION. T2-weighted sequences alone are not sufficient for examining patients with acute flank pain. However, the combined use of both T2-weighted and 3D FLASH sequences will ensure better confidence in the evaluation of acute suspected renal colic. MR urography can replace conventional excretory urography when the latter is contraindicated or undesirable.
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Recently, the use of MR urography using heavily T2-weighted turbo spin-echo sequences such as rapid acquisition with relaxation enhancement (RARE) and half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequences has been described in patients with urinary tract disease [1,2,3,4,5,6,7]. High-resolution images can be achieved with breath-hold and rapid acquisition sequences. HASTE and RARE can show acute urinary obstruction and perirenal high-intensity signals [8]. However, information about renal function is not provided, small stones are difficult to detect, and a nondilated urinary tract is not fully visualized [6, 7, 9]. The use of a paramagnetic contrast agent permits the evaluation of renal excretory function and better visualization of the nondilated urinary tract [10,11,12,13]. However, to our knowledge, there are no prospective clinical studies comparing T2-weighted and rapid gadolinium-enhanced sequences to evaluate patients with suspected acute renal colic.
The aims of this study were to define the clinical value of MR urography in the evaluation of patients with acute flank pain with reference to conventional excretory urography and to compare the diagnostic accuracy and interobserver agreement of heavily T2-weighted (thin-slice HASTE and thick-slab turbo spin-echo) sequences with gadolinium-enhanced three-dimensional fast low-angle shot (3D FLASH) imaging.
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Patients
A total of 40 patients were prospectively evaluated. During the study
period, three patients were excluded. In one patient, the obstruction resolved
during MR urography examination, and a small distal ureteral stone passed into
the urinary bladder (confirmed on sonography). In another patient, 3D FLASH
sequence was not performed because of equipment-related technical
difficulties. In the last patient, MR urography could not be performed because
of the patient's extreme obesity. In addition, three patients refused to
participate in the study. A total of 80 kidneys were examined. Before
presentation to our hospital, the duration of symptoms was less than 72 hr in
34 patients (85%), and six patients (15%) had experienced intermittent
symptoms during a period ranging from 4 to 14 days. Nevertheless, all patients
had acute flank pain symptoms during the 24 hr before presentation to the
emergency department. Data regarding patient characteristics, relevant
laboratory test results, duration of symptoms, and timing and duration of
imaging are presented in Table
1.
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Imaging Methods
MR imaging was performed using a 1.5-T scanner (Magnetom Vision; Siemens,
Erlangen, Germany) with a phased array body coil. Patients were asked to void
before MR urography examination. Otherwise, no specific preparation was
required, and no external compression was applied. Breath-hold sequences were
used. Both two-dimensional T2-weighted MR urography and 3D T1-weighted MR
urography were performed in coronal orientation.
T2-weighted MR urography was performed with thin-slice (fat-suppressed HASTE; TR/TE, 11.90/95; flip angle, 150°; slice thickness, 3-6 mm; matrix, 240 x 256; acquisition time, 15 sec) and thick-slab (fat-suppressed single-shot turbo spin-echo; 2800/1100; flip angle, 150°; slab thickness, 40 mm; matrix, 240 x 256; acquisition time, 7 sec) acquisitions. Field of view was adjusted individually to accommodate different patient sizes. HASTE was also performed in axial orientation (7- to 9-mm slice thickness) to cover the entire abdomen and retroperitoneal space.
T1-weighted MR urography was performed with gadolinium-enhanced 3D FLASH acquisition (4.6/1.8 msec; flip angle, 30°; effective slice thickness, 1.75 mm; field of view, 400 mm; matrix, 200 x 512; acquisition time, 23 sec). Three-dimensional FLASH was also performed in sagittal orientation on the affected side when considered necessary.
A low-dose diuretic injection of 0.1 mg/kg of body weight (total individual dose not exceeding 10 mg) of furosemide (Furesis; Orion, Espoo, Finland) was used to enhance excretion 30-60 sec before the administration of contrast material. Three-dimensional FLASH sequences were routinely repeated 5 and 15 min after the administration of 0.1 mmol/kg of body weight of gadopentatate dimeglumine (Magnevist; Schering, Berlin, Germany), and delayed follow-up was performed when necessary. The total MR urography examination time was calculated starting with the beginning of the first localizing sequence and ending at the acquisition of the last 3D FLASH sequence. The total imaging time of T2-weighted sequences was approximately 6 min, and the total imaging time of all MR urography sequences, if excretion was not delayed, was approximately 25 min.
Maximum-intensity-projection (MIP) images, and occasionally multiplanar reconstruction and original source images, were available on films for evaluation, and both observers had access to source images from the workstation, to review if needed.
Excretory urography was performed with an IV bolus injection of 1.5 mL/kg of body weight (total dose not exceeding 100 mL) of iohexol (300 mg I/mL Omnipaque; Nycomed, Cork, Ireland). No abdominal compression was used. An unenhanced radiograph of the abdomen was initially obtained followed by fullsize radiographs at 5 and 15 min after teh administration of contrast material. Oblique images of the ureterovesical junction were obtained, if needed, to better visualize the distal ureters. The examination was completed if no obstruction was detected. If obstruction was noted, follow-up radiographs on the side of obstruction were obtained at 30 and 60 min, and additional radiographs were obtained as needed until the cause or level of obstruction was identified. The duration of excretory urography examination was calculated starting with the injection of contrast material and ending at the retrieval of the last diagnostic image.
Image Interpretation
The excretory urography examinations were interpreted in consensus by a
radiologist and a urologist for the presence, cause, degree, and level of
obstruction and for the extravasation of contrast material. If the
interpretation of the excretory urography was questionable, or the cause of
obstruction could not be definitely determined, the presence of a ureteral
stone or obstruction was confirmed on helical CT or by passage of the stone.
The largest diameter of a stone, if present, was measured directly from
hard-copy images and was reported in millimeters after subtraction of a
magnification factor of 1.09. After reviewing all patient charts, the final
conclusive diagnosis was made on the basis of a combination of clinical and
imaging results and the findings of interventional examinations or
procedures.
The MR urography investigation and reconstructions were performed by a radiologist who was not involved in analyzing MR urograms. T2-weighted and 3D FLASH sequences were evaluated separately and independently by two experienced radiologists for the presence and cause of obstruction. A stone was defined as a complete or partial filling defect in the urinary tract. Other causes of obstruction, if present, were defined as extrinsic, intrinsic, or indeterminate. The degree of obstruction was assessed subjectively as not present, mild, moderate, or severe. The following criteria were used to assess the degree of obstruction on both excretory and MR urography: no obstruction (no distention of the intrarenal collecting system or ureter; no delay in excretion; absence of columnization [subsequently described]), mild obstruction (the ureter visualized as a persistent column of signal intensity or contrast material, proximal to the level or cause of obstruction on the symptomatic side; mild prominence of the renal pelvis; the whole urinary tract is visualized on 15-min urogram), moderate obstruction (enlargement of the calices with blunting of the caliceal fornices, but the intruding shadows of the papillae, although flattened, are still easily seen; delayed urogram), severe obstruction (increasingly dense nephrogram with markedly delayed excretion; obvious dilatation of the ureter; dilatation and rounding of the calices with obliteration of the papillae).
Also, the presence of extravasation and perirenal high-intensity signal and the level of obstruction were noted. High-intensity signal was subjectively evaluated to be absent, mild, or substantial. The level of obstruction was classified as ureteropelvic junction; proximal, middle, or distal third of the ureter; or ureterovesical junction. The technical quality of MR urograms was judged as good, suboptimal, or inadequate on the basis of the completeness of visualization of the urinary tract and the presence of disturbing artifacts. Observers were aware of the side of the symptoms. No other clinical data or information from other studies were provided, and the observers were not aware of the clinical outcome.
Statistical Analysis
The sensitivity, specificity, and overall accuracy of both T2-weighted and
3D FLASH were calculated for the two observers separately. The kappa statistic
was used to measure interobserver and intertechnique agreement. Strength of
agreement was classified as slight (
0.20), fair (0.21-0.40), moderate
(0.41-0.60), substantial (0.61-0.80), or excellent (0.81-1.0)
[14].
The correlation of high-intensity signal with the degree of obstruction seen in excretory urography was calculated using Sperman's correlation coefficient, which was considered to be significant if the p value was less than 0.05.
Statistical analysis was performed with a PC statistical software package (SPSS version 9.0 for Windows; Statistical Package for the Social Sciences, Chicago, IL).
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In 12 patients the stone passed spontaneously. In three patients the stones were treated with extracorporeal shock wave lithotripsy. Five patients underwent ureteroscopy and stone extraction. In another five patients the stone could no longer be visualized on unenhanced abdominal radiographs or excretory urograms. One patient was asymptomatic on follow-up and refused further evaluation; the passage of stone was not confirmed but the clinical and radiologic diagnosis were of a definite small ureteral stone.
All of the MR urograms were judged to be of good technical quality. Gadolinium-enhanced 3D FLASH MR urography was highly accurate in showing stones as intraluminal filling defects. Observer B correctly diagnosed all cases, but observer A did not detect a ureteral stone in one case. Observer A did not see evidence of a definite stone in 11 patients (42%) on heavily T2-weighted sequences, and observer B did not see such evidence in 12 patients (46%) on heavily T2-weighted sequences. All of the missed stones except one were less than 5 mm in diameter as determined by excretory urography, and the grade of obstruction was mild in most of these cases (in 6 cases for observer A and 7 for observer B). The sensitivity, specificity, overall accuracy, and interobserver agreement values for the detection of ureteral stones and obstruction are shown in Table 2.
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The mean size of detected stones on excretory urography was 4.5 mm, ranging from 2 to 15 mm. When measured from 3D FLASH sequence images, the mean size was 4.1 mm (range, 3-7 mm) for observer A and 3.8 mm (range, 2-10 mm) for observer B. The Spearman's correlation coefficient for stone size compared with excretory urography was 0.48 for observer A (p = 0.02) and 0.37 for observer B (p = 0.06).
The presence of obstruction was correctly diagnosed by both observers with 3D FLASH MR urography in all cases. For T2-weighted sequences, observer A had one false-positive result, and observer B correctly interpreted all cases (Table 2).
As determined by excretory urography, the degree of obstruction was graded
as mild in 10 patients, moderate in 12 patients, and severe in four patients.
The degree of intertechnique agreement with gadolinium-enhanced 3D FLASH MR
urography was substantial for observer A ( = 0.78) and excellent for
observer B ( = 0.88). Interobserver agreement was excellent ( =
0.85). The degree of intertechnique agreement between excretory urography and
T2-weighted sequences was only moderate for both observers (observer A,
= 0.54; observer B, = 0.48). Interobserver agreement for
T2-weighted sequences was substantial ( = 0.64).
The level of obstruction was at the ureteropelvic junction in one, the
proximal ureter in six, the distal ureter in four, and the ureterovesical
junction in 15 patients, as determined by excretory urography. Both the
gadolinium-enhanced 3D FLASH MR urography (observer A, = 0.87;
observer B, = 0.90) and T2-weighted sequences (observer A, =
0.80; observer B, = 0.90) agreed well with the results of excretory
urography in the assessment of the level of obstruction.
Extravasation of contrast media was seen on excretory urography in two patients (7.7% of patients with a proven ureteral stone). Extravasation was clearly seen on 3D FLASH MR urography in these patients, and a third patient was found to have minimal extravasation not visualized on excretory urography.
Both observers found perirenal high-intensity signal to be present in 24
patients (92%) with ureteral stones on T2-weighted images. In two patients
with intermittent symptoms, high-intensity signal was judged to be absent by
both observers. High-intensity signal was also interpreted by observer A to be
positive in two patients and by observer B in three patients, in the absence
of a stone or obstruction. The sensitivity and specificity of perirenal
high-intensity signal in predicting the presence of acute ureteral obstruction
was 96% and 92% and 96% and 89% for observers A and B, respectively.
Interobserver agreement in the assessment of high-intensity signal was
substantial ( = 0.80). The Spearman's correlation coefficient between
the degree of obstruction on excretory urography correlated significantly with
the degree of perirenal high-intensity signal (r = 0.86, p
< 0.001 for observer A; r = 0.84, p < 0.001 for
observer B).
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The difficulty in detection of small ureteral stones with T2-weighted sequences is a known limitation of this technique [2, 4, 6]. Although the exact level of obstruction was readily recognized, in this study the cause of obstruction could not be seen with confidence in many cases (Fig. 1A,1B,1C,1D). The other limitations of heavily T2-weighted sequences include insufficient spatial resolution and the inability to obtain functional information. Caliceal, forniceal, and infundibular anatomy could not be seen with the same detail as in excretory urography [24]. In this context, the use of a paramagnetic contrast agent clearly improves the diagnostic value of MR urography because T1-weighted sequences are capable of revealing even small amounts of gadolinium.
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Contrast-enhanced MR urography is a new concept in imaging patients with urinary tract abnormalities. In his initial experimental study, Nolte-Ernsting et al. [10] presented the advantages of this imaging technique in retrieving high-spatial-resolution images in nonobstructed urinary tracts. Also, in contrast to heavily T2-weighted turbo spin-echo sequences, image quality was not influenced by superimposed abdominal fluid collections. Furthermore, the excretory function of the kidneys could be evaluated. The need for a diuretic to optimize the endoluminal concentration of gadolinium and to produce accelerated distention was addressed. The usefulness of this technique has been evaluated in a clinical study of 71 patients using a respiratory-gated T1-weighted spoiled 3D gradient-echo sequence [11]. The acquisition time, however, was relatively long, ranging from 6 min 39 sec to 8 min 41 sec. T1-weighted sequences with long acquisition times have also been reported in pediatric urogenital imaging [25].
In our study we used a commercially available ultrafast breath-hold 3D FLASH sequence with an imaging time of only 23 sec to generate high-resolution images in the evaluation of patients with acute flank pain (Fig. 2A,2B,2C). Gadolinium-enhanced MR urography proved to be highly accurate in detecting obstruction and determining its cause and level. The results of our study were in concordance with those of Nolte-Ernsting et al. [11], except for higher accuracy in detecting the cause of obstruction. For the detection of ureterohydronephrosis, the sensitivity and specificity were reported to be 90% and 100% [11] with reference to excretory urography. Excellent agreement was also seen in our study for both observers. In the study of Nolte-Ernsting et al., the sensitivity and specificity for the detection of a specific filling defect such as a calculus were 80% and 98%, 64% and 91%, and 84% and 91% for three observers. In the present study both observers identified almost all filling defects correctly. This can be attributed, in part, to our use of a different, ultrafast imaging technique with considerably shorter imaging time, which minimized movement artifacts. Also, we had the advantage of gaining experience with this imaging method on a limited number of volunteers and patients with acute flank pain before this study.
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The present study shows the superiority of gadolinium-enhanced MR urography over static-fluid imaging. Functional information was obtained in the same way as with excretory urography, enabling more accurate evaluation of the degree and cause of obstruction. Three-dimensional FLASH sequence, with its short acquisition time, permits repeated series of breath-hold images in optimal orientations, thus providing obvious improvement to the methods previously described by other investigators. The whole urinary tract, including nondilated ureters, can be visualized. The high-resolution images allow good reconstructed image quality. High-quality MIP images and multiplanar reconstructions are of use especially in evaluating the distal ureters. MIP images resemble those of excretory urography, with better spatial resolution than HASTE or thick-slab turbo spin-echo sequences. Also, extravasation can be clearly seen (Fig. 3A,3B). During this study we were able to visualize the fine anatomic details of the pelvicaliceal system with 3D FLASH sequences (Fig. 4A,4B,4C,4D), and in some patients we saw caliceal diverticula and papillary blush. We did not, however, conduct systematic side-by-side comparison with excretory urography for these details.
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Several issues, however, are to be considered with this new imaging technique. Gadolinium-enhanced MR urography cannot be recommended for the evaluation of pregnant women and is of no use in the evaluation of the urinary tract on the side of the nonexcreting kidney. Furthermore, in cases of severe obstruction and delayed excretion, the long duration of excretory MR urography for revealing the underlying abnormality increases patient and personnel inconvenience. On the other hand, the imaging time is not significantly longer than that of excretory urography. In the present study, if excretion was delayed, the patient was removed from the magnetic tube and the examination was repeated at different intervals, as needed, until the level and cause of obstruction were visible. In this study, 3D FLASH MR urography seemed to slightly underestimate the size of stones, a fact that should be taken into consideration when planning for conservative treatment. Breath-hold techniques also require cooperative patients; therefore, severely ill patients, infants, small children, and possibly the elderly may fail to comply, which results in somewhat poorer image quality.
Unfortunately, our patient population did not include obstructive disease entities other than ureteral stones; this exclusion can be considered a limitation of this study. However, in our prospective study the patient material was consecutive and represents clinical reality at our institution.
The false-positive interpretation of perirenal high-intensity signal, in a minority of cases, was suggestively attributed to chemical-shift misregistration artifacts. Otherwise, misregistration and pulsation artifacts, occasionally noted in all sequences, were not found to interfere with the interpretation of source images. Occasionally, we noted turbulence-induced artifacts in the pelvicaliceal system, probably caused by diuretic-induced diuresis, especially in the first excretory acquisitions. These artifacts were not visible on subsequent sequences. In some cases, we were also able to see signal void lines in the urinary bladder originating from the ureteral orifices and caused by a ureteral jet, which we found to be an additional useful sign in excluding obstruction (Fig. 5A,5B).
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The amount of diuretic used was relatively small but was considered sufficient to optimize image quality. However, one patient with severe obstruction caused by a large stone had sudden exacerbation of acute flank pain symptoms requiring analgesic medication immediately after diuretic injection, which we considered to be a complication. The pain rapidly subsided, and the patient continued with the MR urography examination.
T2-weighted sequences revealed renal, ovarian, or hepatic cysts in 12 patients. A large gallbladder stone was also visualized in one patient. These findings were believed to be of no acute clinical value. In one patient, the cause of acute pain was attributed to gallbladder stones clearly visualized on T2-weighted images. The nondilated common bile duct with no filling defects was also visible. These findings were later confirmed using sonography and surgery. In another patient, mild right-sided colonic and pericolic edema with a small amount of ascites were noted by both observers. A possible colitis was suggested. The final diagnosis was ulcerative colitis proved by biopsy.
The economic aspects of MR urography have been discussed in detail in the editorial comments by Hattery and King [24]. We agree that emphasis must be first on the patient and the quality of care, and second on economics. Nevertheless, the high cost of MR urography and its limited availability play major roles in restricting its wide use. We believe that the selection of patients with clinically atypical symptoms, and the simultaneous use of other sequences immediately after gadolinium injection to examine the whole abdomen, thus providing a complete one-step diagnostic workup, could justify the use of MR urography. At our hospital, the calculated cost of contrast-enhanced MR urography is approximately $500, compared with $97 for excretory urography and $185 for unenhanced CT. In our practice, we now recommend the use of contrast-enhanced MR urography for cooperative children and young adults. By reducing the field of view to a minimum, we are able to reduce the imaging time of 3D FLASH to 15 sec, which seems to be tolerable to some older children.
Unenhanced CT is becoming increasingly accepted as the first imaging "test of choice" in the evaluation of patients with acute flank pain. CT is shown to be a safe, rapid, and highly accurate technique in the detection of ureteral stones, allows precise determination of stone size and location, and also shows secondary signs of obstruction [26]. CT also enables the diagnosis of a wide range of disease entities that can result in flank pain and can confidently exclude many serious diseases. A well known limitation of this technique is the difficulty in differentiating distal calculi from pelvic phleboliths [27]. Also, the exposure to ionized radiation is higher than with urography [28]. In this context, children and young adults will benefit most from the MR urography advances. In obstetric patients, T2-weighted sequences might provide sufficient information and thus eliminate the need for the use of contrast material and ionized radiation. Other future roles of MR urography could include use in showing the anatomic details of the pelvicaliceal systems and ureters and use in the functional evaluation of kidneys, especially in patients with kidney failure or contrast allergy. Another advantage of MR urography might be the evaluation of abdominal organs in cases of atypical symptoms with suspicion of an inflammatory process, due to the superiority of T2-weighted sequences in revealing edema and even small amounts of ascites.
In conclusion, MR urography is a valuable imaging technique for examining patients with acute flank pain. T2-weighted sequences can rapidly reveal the presence of perirenal high-intensity signal, obstruction, and level of obstruction, and thus rapidly provide information for diagnosing the urinary tract abnormality. The addition of gadolinium-enhanced sequences significantly increases the diagnostic efficacy of MR urography. Further investigations are needed to evaluate the usefulness of MR urography in comparison with the diagnostic value of helical CT in the examination of patients with acute flank pain.
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